Our present invention pertains generally to the field of lightning protection means for aircraft and other vehicles. More particularly, the invention relates to lightning protection means for composite structures on aircraft or other vehicles.
Advanced (boron and graphite epoxy) composite materials are being increasingly used to replace conventional metals in various aircraft structural components since significant weight savings can be achieved. These composite materials are not electrically nor thermally as conductive as conventional metals, however, and lightning strikes on composite components can seriously degrade the structural integrity of such components. The weight factor is the most important design consideration for aircraft composite structures, and certain prior lightning protection systems of others are not applicable to composite aircraft structures due to the unacceptable weight penalty imposed.
In our copending U.S. patent application Ser. No. 401,388 filed Sept. 27, 1973 for Aircraft Lightning Protection System, now U.S. Pat. 3,906,308 there is disclosed and claimed a lightning protection system utilizing dielectric materials in the form of coatings and/or films applied to critical composite or metallic components located at the external surface of an aircraft to provide a complete dielectric shield which protects the components from lightning channel attachments. For large span skin panel protection, properly oriented and spaced foil or solid metal strips of appropriate dimensions are additionally affixed to the surface of the dielectric material on the skin panel and grounded to provide dwell points for the lightning channel.
While the aircraft lightning protection system described and illustrated in the U.S. application Ser. No. 401,388 identified above is fully applicable for use on composite aircraft components, there are some composite components which have certain characteristics or requirements that may make such lightning protection system difficult to use or even prohibit its use. For example, a composite component may have unusually sharp contours and high curvatures in more than one direction so that a desired dielectric film shield cannot be easily applied correctly to the surface of the component. A composite skin panel may have antenna ground plane or electromagnetic interference (EMI) and electromagnetic pulse (EMP) protection requirements which cannot be met by the noted lightning protection system.
Closely related to the structural integrity of aircraft composite structures are their connecting joints. Structural joints are clearly fundamental elements of the structure of an aircraft. It is vitally important that the strength of these structural joints not be degraded during a lightning strike incident. The conventional metal-to-metal structural joints have relatively low resistance and are not normally vulnerable to lightning damage from transferring lightning current through such joints. This is not the case, however, for the composite-to-metal and composite-to-composite structural joints. There is presently no known means, system, or process in the industry which can be used with, or applied to, composite-to-metal and composite-to-composite structural joints to reduce their vulnerability to lightning strike damage.